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During the second trimester, 10–20 percent of the total proteins of AF appear to be of intestinal origin.¹⁸¹ Developmental patterns of AF disaccharidases have been studied by Potier et al.¹³⁰ and Antonowicz et al.¹³¹ With the exception of trehalase, all disaccharidase activities vary approximately in parallel during pregnancy, being present from 10 weeks, reaching maximum values around 15–18 weeks, and dropping rapidly to low values after 22 weeks (Figure 3.3). In contrast, trehalase activity increased after 22 weeks.

Figure 3.3 A comparison of the various disaccharidase activities in amniotic fluid between 14 and 42 weeks of gestation, showing the different patterns of trehalase activity. All activities are expressed as a percentage of the mean activity at 14–17 weeks of gestation. — ‐ — ‐ Maltase, ‐ ‐ ‐ sucrase, – – – trehalase, —— palatinase, . . . lactase.

The drop of disaccharidase activities at around 22 weeks is due to increasing fetal swallowing with age and accumulation of large quantities of meconium in the fetal intestine.^{182, 183} Similar developmental patterns were observed for intestinal alkaline phosphatase and trypsin.^{184, 185} Results of studies on animal models support the conclusion that there is a relationship between meconium accumulation in the fetal intestine and the rapid fall of disaccharidase activities in the AF.¹⁸²